In recent years, much effort has been put into the incorporation of carbon nanotubes in polymer matrices. The composites obtained are interesting materials, since they have enhanced electrical and mechanical properties at very low loading due to the specific nanotube characteristics, such as their high aspect ratio and electrical conductance. However, dispersion of carbon nanoutubes in highly viscous polymers is difficult and has often been attempted by functionalizing the nanotubes, leading to attractive interactions between the nanotubes and the polymer. In addition, dispersing exfoliated single nanotubes has been found to be a challenge, since nanotubes are highly bundled as a result of strong van-der-Waals interactions.
In general, materials can be divided into three groups regarding their electrical conductivity δ: insulators (δ<10−7 S/m), semi-conductors (δ=10−7−105 S/m) and conductors (δ>105 S/m). For polymers, typical conductivity values range from 10−15 S/m up to 10−12 S/m. Carbon fillers can have conductivities in the range of 104 S/m up to 107 S/m. In composites, the conductivity levels off to a slightly lower value than for the pure carbon species at higher filler concentration.
Carbon nanotube reinforced polymers are presently made by incorporating carbon nanotubes (CNTs), generally in the form of a bundle, in a polymer matrix. In order to obtain a homogeneous distribution of these CNTs, they are pretreated by either an ultrasonic treatment, or by a chemical modification process, aimed at improving the dispersability of the individual CNT in the polymer matrix. The incorporation of CNTs in such a polymeric matrix is for the enhancement of the stiffness as well as the conductivity of the polymer matrix material.
The reported procedures for obtaining homogeneous dispersions of CNTs in polymer matrices result in either breaking and lowering of the aspect ratio of the tubes (which is unfavourable for stiffness, strength, and conductivity of the composite), or in damaging the surface of the tubes (which lowers the stability and the conductivity of the tubes).
The process of the present invention offers a solution to this problem, as a result of which the CNTs remain substantially of the same length and aspect ratio. The reinforced polymer resulting from the process of the present invention has enhanced conductive and mechanical properties.
In J. Mater. Sci, 37, 2002, pages 3915-23, a process is described for the preparation of a poly(styrene/butyl acrylate) copolymer nano composite using CNTs as filler. This process uses multiwall CNTs (MWNT), suspended in an aqueous solution of sodium dodecyl sulphate (SDS), and a latex of the copolymer. An amount of at least 3 wt. % of the MWNT is needed to have a significant change in the electrical conductivity of the nanocomposite (the so-called percolation threshold).